By structure is meant a finished material body, homogeneous or heterogeneous, designed to resist predetermined physical stresses which will be called "load", simple or combined, the invention seeking to predict the limit of resistance of said body relative to said charge.
Although the invention is susceptible to uses in many structures, it will be described as to its application for the inspection of storage reservoirs of gas under high pressure and more particularly still to the inspection of reservoirs made of composite material wound on a metallic liner.
Such reservoirs, generally spherical, adapted to resist internal pressures which can reach or exceed 800 bars, are subjected to non-destructive inspection among which is inspection by acoustic emission.
This technique, which seeks to inspect the soundness of the metal and of the composite material, consists in subjecting the reservoir to be tested to a predetermined applied pressure, which gives rise in the materials to microscopic irreversible damage whose appearance releases energy in thermal or acoustic form. Only the acoustical energy, which is more easily usable, is detected by the aid of piezoelectric detectors and permits detecting any incipient fault.
Thus, in the course of a test cycle by a monotone increasing from one pressure to a maximum predetermined pressure, typically the rated pressure, which is to say 1.5 times the service pressure of said reservoir, the acoustic emissions are recorded.
The use of these acoustic emissions permits, starting from preestablished criteria, certifying the reservoir to be safe or unsafe to fulfill its purpose.
It is evident that this technique, if it permits detecting or even localizing in the reservoir under test the faults, gives no indication of the effective value of the pressure at rupture, necessarily beyond the rated pressure.
Until now, there existed no inspection that quantifies for each reservoir the foreseeable level of pressure at rupture, which requires, for the sake of safety, using safety coefficients leading inevitably to overdimensioning. However, this overdimensioning gives rise in particular for structures of costly material, such as the reservoirs described above, for aeronautical or space use, to increases of weight and of undesirable cost.
So as to reduce this overdimensioning whilst increasing the safety of use of such structures, the applicant has already proposed in French FR-A-2 715 731, a non-destructive technique for the precise individual evaluation of each structure as to its effective limit of resistance to stresses which must be borne and in consideration of which it has been designed and produced.
In the field of pressure resistance of structures such as reservoirs of composite material wound on a metallic liner, one generally distinguishes three regimes in the course of increase of monotone pressure from zero to rupture of the structure, with the rupture pressure designated Pr.
Up to a value of pressure, very roughly fixed at 0.7 Pr, is located a so-called diffuse regime in which there is no display in the structure under test of interactions or correlations between eventual damage, such that no law can be established linking such damage, which renders impossible any prediction of the value Pr.
Beyond this first regime, and up to a value equal approximately to 0.95 Pr, there is a second regime called pre-critical, in which appear interactions between damage, having a spatial and temporal coherence. This is the beginning of a so-called cooperative process.
Finally, beyond the pre-critical regime and up to rupture of the structure, there is a third so-called critical regime, in which the interactions between damage are generalized and, by said cooperative process, lead to destruction of the structure.
In FR-A-2 715 731 there is described a process for the predictive determination of the load at rupture, using a power law taking account of the correlations between damage but essentially when these latter are in the totally cooperative process leading to rupture. Stated otherwise, the procedure cannot give really pertinent and reliable data as to the load at rupture of the structure under test except in the critical regime, which requires bringing the test pressure of the structure into the region near the rupture pressure.
However, this can give rise to a decrease in the rupture pressure Pr because of reaching a test pressure relatively near Pr is adapted to give rise to damage rendering the structure fragile, which is to say having consequences on the strength under pressure, thus lowering the value of Pr.
It is therefore desirable to reduce as much as possible the maximum test pressure for each structure.